Warn when pil_kwargs["pnginfo"] overrides metadata.

anntzer · anntzer · commit c212f2fe79c7 · 2020-05-28T09:44:03.000+02:00
diff --git a/lib/matplotlib/backends/backend_agg.py b/lib/matplotlib/backends/backend_agg.py
@@ -498,14 +498,6 @@ def print_png(self, filename_or_obj, *args,
             If the 'pnginfo' key is present, it completely overrides
             *metadata*, including the default 'Software' key.
         """
-
-        if metadata is None:
-            metadata = {}
-        metadata = {
-            "Software":
-                f"matplotlib version{mpl.__version__}, http://matplotlib.org/",
-            **metadata,
-        }
         FigureCanvasAgg.draw(self)
         mpl.image.imsave(
             filename_or_obj, np.asarray(self.buffer_rgba()), format="png",
diff --git a/lib/matplotlib/image.py b/lib/matplotlib/image.py
@@ -1520,14 +1520,22 @@ def imsave(fname, arr, vmin=None, vmax=None, cmap=None, format=None,
         pil_shape = (rgba.shape[1], rgba.shape[0])
         image = PIL.Image.frombuffer(
             "RGBA", pil_shape, rgba, "raw", "RGBA", 0, 1)
-        if (format == "png"
-                and metadata is not None and "pnginfo" not in pil_kwargs):
+        if format == "png":
             # Only use the metadata kwarg if pnginfo is not set, because the
             # semantics of duplicate keys in pnginfo is unclear.
-            pnginfo = PIL.PngImagePlugin.PngInfo()
-            for k, v in metadata.items():
-                pnginfo.add_text(k, v)
-            pil_kwargs["pnginfo"] = pnginfo
+            if "pnginfo" in pil_kwargs:
+                if metadata:
+                    cbook._warn_external("'metadata' is overridden by the "
+                                         "'pnginfo' entry in 'pil_kwargs'.")
+            else:
+                metadata = {
+                    "Software": (f"Matplotlib version{mpl.__version__}, "
+                                 f"https://matplotlib.org/"),
+                    **(metadata if metadata is not None else {}),
+                }
+                pil_kwargs["pnginfo"] = pnginfo = PIL.PngImagePlugin.PngInfo()
+                for k, v in metadata.items():
+                    pnginfo.add_text(k, v)
         if format in ["jpg", "jpeg"]:
             format = "jpeg"  # Pillow doesn't recognize "jpg".
             facecolor = mpl.rcParams["savefig.facecolor"]
